1. Field of the Invention
The present invention relates generally to an apparatus and method for developing latent electrostatic or magnetic images, and is particularly concerned with an apparatus and method for carrying out magnetic brush development of latent images on vertically presented recording surfaces.
2. Description of the Prior Art
In conventional xerographic and electrostatic recording processes, a latent charge image is formed on a recording surface and is then developed into a visible image by applying a pigmented developer material. The recording surface may consist, for example, of a photoconductive layer which is initially provided with a uniform electrostatic charge. The photoconductive layer is then selectively discharged in an imagewise manner by exposing the recording layer to a light pattern corresponding to the image to be reproduced. This produces a latent electrostatic image to which oppositely charged developer particles will adhere. The developed image can be fixed or rendered permanent in various ways, such as by applying heat, pressure, solvents, and combinations of these.
The foregoing is essentially an optical image reproduction process, and is employed in most types of commercially available document photocopying machines. The photoconductive layer may be provided on the final recording medium, as in the case of coated paper xerography, or it may be provided on the surface of an intermediate transfer member such as a rotary drum. The latter arrangement is employed in xerographic copiers of the plain paper type.
Electrostatic printing techniques have also been developed in which latent charge images are created non-optically on a dielectric charge retentive surface by means of an electrostatic print head, which is typically of the dot matrix type. The dielectric layer, like the photoconductive layer in optical copiers, may be provided either on the final recording medium or on an intermediate transfer member. In non-optical electrostatic printing systems, however, the dielectric layer is not required to be light-sensitive.
Various methods have been employed for developing (i.e., rendering visible) the latent charge images created by xerographic and electrostatic imaging techniques. One early developing method involved cascading the developer material across the latent image areas to be developed. Another method, referred to as powder cloud development, involved dispersing the developer particles in a moving stream or flow of air and then bringing the entrained particles into contact with the latent image bearing surface. Rotating fur brushes were also used to apply the developer particles to the recording surface in some early types of xerographic and electrostatic imaging apparatus.
A more common developing method at the present time is referred to as magnetic brush development. This involves the use of a magnetic element, typically in the form of a cylindrical roll, for carrying the developer material and applying it to the latent image bearing surface. The developer material may be of the two-component type, in which finely divided and pigmented toner powder is interspersed with somewhat larger ferromagnetic carrier particles. Alternatively, the developer material may be of the single-component type, in which only one kind of particle is involved. A common type of single-component developer consists of fine particles of magnetic material, such as iron or iron oxide, encapsulated within a resin having a relatively low softening temperature. A suitable pigment such as carbon black is usually added to the resin in order to impart the desired color to the developer material.
When placed in a magnetic field, a developer material of either the two-component or single-component type will form streamers resembling the bristles of a brush, similar to the way in which iron filings will align themselves with the magnetic flux lines at the ends of a bar magnet. This property is exploited in magnetic brush developing systems by utilizing a magnetic roll assembly to retain a brush-like layer of developer material on its peripheral surface. The layer of developer is brought into light brushing contact with the latent image bearing surface, which is usually moving in a direction normal to the roll axis as the roll itself rotates. The brushing action brings the developer material into intimate contact with the recording surface and permits electrostatic transfer of the developer particles from the roll to the latent image areas.
A number of different structural configurations have been employed in magnetic brush developing systems. The simplest arrangement is an exposed magnetic roll which carries a layer of developer material on its peripheral surface. The roll may be magnetized in various ways, either intrinsically or by covering the peripheral surface of the roll with a magnetic material. An alternative arrangement, more widely used at the present time, is a two-part roll assembly consisting of an inner magnetic element enclosed within an outer non-magnetic shell or sleeve. The shell is usually cylindrical in shape and provides a smooth carrier surface over which the developer particles can slide while being held by the inner magnetic element. The magnetic flux density at the shell surface will be a function of the spacing between the shell and the inner magnetic element, and of the magnetic permeability of the shell material. Hence by appropriate selection of these factors, it is possible to obtain close control over the magnetic field strength that is used to hold the developer particles on the surface of the shell. Another advantage of the shell is that it provides a useful barrier against contamination of the inner magnetic element and any associated bearings, shafts, and the like with developer particles.
Various types of two-part magnetic brush rolls have been proposed. In one form of the device, the inner magnetic element rotates while the outer shell is held stationary. The rotation of the inner magnetic element causes a backward tumbling or somersaulting motion of the developer particles on the outside circumference of the shell, resulting in a net propagation of the developer material in the direction opposite to the rotational direction of the inner magnetic element. The propagation rate of the developer particles is much less than the rotational speed of the inner magnetic element, but is sufficient to assure a continuous flow of developer particles to the developing zone. In another form of the device, the outer shell rotates with respect to the inner magnetic element which is held stationary. This embodiment is usually used with two-component developers, since the rotation of the outer shell induces thorough mixing between the toner and carrier particles and continuous replacement of spent developer (i.e., denuded carrier particles) at the developing zone.
In embodiments wherein a rotatable shell is employed, it is possible to control the rate of movement of the developer material by varying the rotational speed of the shell. Hence it is possible to deliver more developer material to the developing zone by increasing the rotational speed of the shell, and conversely, less developer material is carried to the developing zone when the shell speed is reduced. In fixed-shell embodiments, a similar but less pronounced effect can be obtained by varying the speed of the inner magnetic element.
In some arrangements, a second magnetic roll, referred to as a supply roll, is positioned between the main developing roll and a source or reservoir of developer particles. The supply roll delivers a metered amount of developer material to the main roll and provides further mixing or agitation of the developer particles.
In the design of magnetic brush developing systems, it is conventional to mount the magnetic brush roll horizontally, that is, with its axis of rotation oriented in a horizontal direction. Such mounting of the magnetic brush roll may be dictated by the path of movement of the recording medium and by other factors as well. For example, it is a relatively simple matter to supply developer particles in a uniform manner to a horizontally positioned magnetic brush roll, as by partially immersing the magnetic brush roll (or an associated supply roll) in a horizontal bed or trough of developer particles. This would not be feasible with a non-horizontal roll. Horizontal positioning of the magnetic brush roll also insures that a uniform layer of developer particles is maintained on the outside surface of the roll. A non-horizontal roll, on the other hand, would be subject to gravity-induced creep of the developer particles in a downward direction along the axial length of the roll.
In most instances, a horizontally positioned magnetic brush roll is entirely acceptable and may in fact be required by the path of movement of the recording surface to which the developer material must be applied. In this connection, it is usually desirable to position the magnetic brush roll with its axis of rotation transverse to the direction of movement of the recording surface, so that the layer of developer material is brushed uniformly across the width of the recording surface. It follows that a horizontally positioned magnetic brush roll is capable of developing a recording surface only when the recording surface has its lateral or transverse dimension (i.e., the dimension perpendicular to its direction of movement) extending in a horizontal direction. In some applications, however, it is desirable to orient the recording surface so that its lateral or transverse dimension extends in a vertical direction. This might occur, for example, when the recording medium comprises a paper web which is held in a vertical plane and moved in a horizontal direction. An equivalent situation occurs in the case of a drum-type recording member when the drum has its axis of rotation disposed vertically. In these instances, it is not feasible to employ a horizontally positioned magnetic brush roll to develop the latent images on the recording surface.
In U.S. Pat. No. 3,777,707, to Robert James Hodges, a magnetic powder handling arrangement is described which is said to be capable of applying magnetic powder to a latent image formed on a vertically held surface. The disclosed apparatus consists of a roller having its axis vertical, with a curved shield embracing the roller. The outside surface of the roller and the inside surface of the curved shield are each faced with a sheet of magnetic rubber which is magnetized in strips. In the case of the roller, the strips extend parallel to the axis of rotation, while in the case of the curved shield the strips are arranged helically. The lower ends of the roller and shield are both immersed in a reservoir of magnetic powder. Rotation of the roller within the curved shield causes the powder to be drawn upward within the annular gap which exists between the outside surface of the roller and the inside surface of the shield, due to the interaction between the parallel magnetic strips on the roller and the intersecting helical magnetic strips on the shield. A vertical gap is provided in the curved shield in order to allow the powder layer on the inner roller to make contact with a vertically held surface on which a latent image has been formed.
Although the arrangement described in U.S. Pat. No. 3,777,707 is theoretically capable of applying magnetic developer particles to a vertically held surface, several practical problems with this device are immediately apparent. For example, since powder is supplied only to the lower part of the roller, the effect of gravity is likely to cause the powder layer to be thicker at the bottom of the roller than at the top, despite the lifting effect of the intersecting magnetic strips. It is also apparent that the thickness of the powder layer on the inner roller will inherently be fixed by the size of the gap between the inner roller and the outer shield, and will not be capable of adjustment except by modifying the overall dimensions of the device. If the gap is made too small, the movement of the powder between the roller and shield may be impeded. If the gap is made too large, the necessary interaction between the magnetic strips on the outer surface of the roller and the intersecting strips on the inner surface of the shield may be reduced to a point where the powder is no longer drawn upwardly. Another problem with the device is that no provision is made for limiting the amount of magnetic powder which is drawn upward between the inner roller and outer shell. Therefore, to the extent that the amount of powder which is transported upward exceeds the amount transferred to the latent image bearing surface, the excess powder would theoretically overflow the top of the device. Finally, the Hodges apparatus places the magnetic powder in direct contact with the magnetic strips on the inner roller and the outer shield, and the device therefore lacks the advantages associated with other types of magnetic brush developing apparatus in which the developer layer is carried by a separate non-magnetic shell in surrounding relationship with the inner magnetic element.
Although the foregoing discussion has been concerned primarily with the development of latent electrostatic images, it should be pointed out that similar considerations apply to magnetic imaging systems as well. Magnetic imaging can be carried out by magnetizing selected areas of a layer of magnetic material using a magnetic recording head. Alternatively, imaging can be accomplished by imparting uniform magnetization to a layer of magnetic material, and then selectively demagnetizing the material in an imagewise pattern by raising the temperature of selected areas above the Curie point of the material (e.g., by a flash exposure). Both methods leave the layer of material with a latent magnetic image which can be rendered visible by the application of a magnetically attractable developer material. Magnetic brush developers can be used for this purpose, as long as steps are taken to avoid distortion of the magnetic image by the magnetic field produced by the developer roll.